198. A Rotifer.
While the Crustacea may have resulted from a series of prototypes leading up from the Rotifers (Fig. 198), it is barely possible that one of these creatures may have given rise to a form resulting in two series of beings, one leading to the Leptus form, the other to the Nauplius. For the true Annelides (Chætopods) are too circumscribed and homogeneous a group to allow us to look to them for the ancestral forms of insects. But that the insects may have descended from some low worms is not improbable when we reflect that the Syllis and allied genera of Annelides bear appendages consisting of numerous joints; indeed, the strange Dujardinia rotifers, figured by Quatrefages, in its general form is remarkably like the larva of Chloëon. It has a quite distinct head, bearing five long, slender, jointed antennæ, and but eight or nine rings to the body, which ends in two long, many jointed appendages exactly like the tentacles. Quatrefages adds, that its movements are usually slow, but "when it wishes to move more rapidly, it moves its body alternately up and down with much vivacity, and shoots forwards by bounds, so to speak, a little after the manner of the larvæ of the mosquito" (Histoire Naturelle des Annelés, Tome 2, p. 69). The gills of aquatic insects only differ from those of worms in possessing tracheæ, though the gills of the Crustacea may be directly compared with those of insects.
But when once inside the circle of the class of insects the ground is firmer, as our knowledge is surer. Granting now that the Leptus-like ancestor of the six-footed insects has become established, it is not so difficult to see how the Poduræ and finally a form like Campodea appeared. Aquatic forms resembling the larva of the Ephemeræ, Perlæ and, more remotely, the Forficulæ and white ants of to-day were probably evolved with comparative suddenness. Given the evolution of forms like the earwigs (Forficula), cockroaches and white ants (Termes), the latter of which abounded in the coal period, and it was not a great step forward to the evolution of the Dragonflies, the Psocus, the Chrysopa, the lice or parasitic Hemiptera, together with Thrips, thus forming the establishment of lines of development leading up to those Neuroptera with a complete metamorphosis, and finally to the grasshoppers and other forms of Orthoptera, together with the Hemiptera.
199. Chrysopa.
200. Panorpa.
We have thus advanced from wingless to winged forms, i. e., from insects without a metamorphosis to those with a partial metamorphosis like the Perlas; to the May flies and Dragon flies, in which the adult is still more unlike the larva; to the Chrysopa (Fig. 199) and Forceps Tails (Panorpa, Fig. 200) and Caddis flies, in which, especially the latter, the metamorphosis is complete, the pupa being inactive and enclosed in a cocoon.
